The incorporation of zinc compounds into oral care products to provide anti-plaque effects, deriving from the antimicrobial properties of the zinc, is well known in the prior art. Also well known is the astringency of such compounds, which produces an unpleasant taste in the mouth and is an inhibition to their use in mass appeal products. This also imposes some restrictions on the flavours that can successfully be used in a zinc containing oral composition.
It has previously been recognised that the more soluble zinc salts, such as zinc chloride and zinc nitrate generally give rise to a worse taste than the less soluble zinc salts, such as zinc oxide and zinc citrate dihydrate. It has also been generally believed that the zinc needs to be in soluble form to be efficacious against bacteria and plaque. There is therefore a trade-off to be made between efficacy and taste. Many attempts have been made, and described in the patent literature, to reduce the astringency of zinc in oral compositions, especially dentifrice compositions. A non-exhaustive list includes the following disclosures.
WO 94/14406 and WO 94/14407 describe formulating a source of zinc ions, preferably zinc oxide or zinc nitrate, along with sources of citrate and pyrophosphate ions in defined ratios.
WO 94/26244 and WO 94/26245 report surprising anti-plaque effectiveness from oral care compositions comprising insoluble zinc oxide, even from compositions having a pH of about 7.5-9.5 through the addition of sodium bicarbonate. The compositions are also absent the metallic and astringent taste of zinc. WO 94/26244 hypothesises that the anti-plaque activity arises through the release of zinc ions by the acidic action of plaque acids on zinc oxide trapped in the plaque.
WO 95/34274, acknowledging the teaching of WO 94/26244, goes further and discloses that sodium bicarbonate itself reduces the astringency of zinc species other than the oxide, such as zinc citrate or zinc chloride.
WO 96/25913 describes the preparation of monophasic zinc hydroxycarbonate from the reaction of a soluble zinc salt with an alkali metal carbonate. WO 96/25913 further discloses that zinc hydroxycarbonate shows synergistic antimicrobial activity in toothpastes with abrasive agents and that lactic acid generated in the mouth triggers the release of zinc ions.
WO 96/37183 discloses that a combination of a humectant and a salt, such as the chloride, carbonate or bicarbonate of sodium or potassium, can mask astringent tastes, such as those of zinc chloride, zinc salicylate, zinc gluconate, silver nitrate, and silver gluconate.
WO 98/37859 also advises the salt approach, teaching that sodium or ammonium chloride as well as alkali, alkaline earth or ammonium carbonates mask the taste of easily soluble zinc salts, especially the chloride, sulfate, lactate and acetate.
A different approach is offered by WO 99/20238, whose invention employs a nonionic polymer to mask the astringency of zinc chloride, sulfate, citrate, acetate and thiocyanate.
The invention of WO 00/28952 is to mask the undesirable taste of an ionisable zinc compound, such as the oxide, citrate, chloride, acetate, hydroxide, fluorosilicate, fluorozirconate, acetate, gluconate with a synergistic flavouring combination of a flavouring oil and lauryl alcohol.
EP 1 072 253 A1 reports that the astringency of zinc oxide is reduced by decreasing its particle size and thereby increasing its surface area, a further taste enhancement is obtained by combining it with Palatinit (isomalt).
US 2003/0202945 uses a physical approach to the zinc taste problem, incorporating its zinc salts, especially zinc citrate dihydrate, into its oral products in the form of relatively large beads.
Whilst the foregoing disclosures primarily aim to mask the taste of zinc, WO 00/61092 increases the bioavailability of zinc in a dentifrice, thereby enabling a reduction in the amount, by buffering the dentifrice at a pH of from 3 to 5.5.
Despite all of the foregoing, further improvements are needed in the formulation of zinc into oral compositions, in order to deliver the combination of anti-plaque efficacy with acceptable taste.
It has now surprisingly been found that carbonate ions appear to potentiate the action of zinc but that the best aesthetics are obtained when the level of carbonate is relatively low compared to the zinc. Without wishing to be bound by theory it is believed that the carbonate ion acts to make the zinc ion bioavailable in terms of its anti-plaque efficacy, releasing it as necessary and drawing upon the insoluble zinc oxide as reservoir of potential zinc ions. This is somewhat surprising given that zinc carbonate itself is relatively insoluble. It has been found by the present inventors that if too much carbonate is used the taste of the product is affected and the zinc taste becomes more noticeable. This is believed to be due to making too much zinc ion available.